Common drain dual MOSFET power devices are mainly used in the management of battery charging and discharging in notebook computers, Tablets or mobile phones. As the recent development is to achieve a shorter charging time, the charge and discharge current needs to increase correspondingly, and the on-resistance of the device needs to be further reduced. In addition, the ultra-thin design of tablets and mobile phones also requires the dimensions of the power devices to be minimized.
FIG. 1A shows a conventional packaging structure for a power MOSFET. In the top view of FIG. 1A, a lead frame 101 is provided and a semiconductor chip 102, with a plurality of solder balls formed on its front, is attached to the lead frame 101, then after curing and molding steps, the front of the packaging layer is ground to expose the solder balls. However, the use of the lead frame 101 increases the manufacturing cost and the thickness of the device. In general the lead frame is required to withstand a high clamping pressure in the molding step; otherwise it may easily be deformed and damaged under higher clamping pressure. Further, for the packaging structure shown in FIG. 1A, the chip 102 is attached on the lead frame 101 using a standard SMT process, thus this package is not a true chip-scale package and the cost is high due to the complicated process.
As shown in cross-sectional view of FIG. 1B, a metal layer 121 is deposited directly on the back of the chip 122 without using a lead frame. However, marking on such exposed metal layer as 121 is more difficult and is easy to wear off, and the metal layer 121 is too thin (in the range of a few microns) to reducing the on-resistance or other types of parasitic resistance of the device. In cross-sectional view of FIG. 1C, a plastic layer 123 is formed on the metal layer 121. In this case, the marking can be done by printing on or etching the plastic layer 123. However, in order to achieve chip scale packaging, polishing and thinning of plastic layer 123 are needed; as such the surface of the ground plastic layer 123 can be a rough surface with pits, which leads to a marking issue.
It is within this context that embodiments of the present invention arise.